This invention relates to a scanner used for ultrasonic inspection of pipings in a system such as an atomic power plant, and more particularly to an improvement of a trackless scanner which need not travel on any tracks for its movement, the improvement being such that the trackless scanner can be prevented from being deviated circumferentially of a piping so as to stably scan straight and curved pipe portions.
Japanese Patent Unexamined Publication No. 58-129253 discloses a prior art scanner adapted for scanning straight and curved pipe portions. This prior art scanner has curvature detectors located before and behind a drive wheel which runs in an axial direction of a piping. A curvature of the piping is measured from amounts of expansion and contraction of these curvature detectors and used for controlling posture of the scanner such that the scanner can travel along the straight and curved pipe portions. The scanner of this prior art is efficient to travel in the axial direction of a piping to be examined but due to lack of a function to detect deviated turning of the scanner about the piping axis and a means for preventing the circumferential deviation, this scanner is liable to travel along the surface of the piping spirally or in a snaking fashion including alternate clockwise or rightward and counterclockwise or leftward turnings. This circumferential turning of the scanner is considered to be attributable to a complicatedly related interaction between clearance for a shaft of the drive wheel, accuracy of mounting the drive wheel and unevenness of ground pressure of the drive wheel. When a piping to be examined consists of only horizontally laid pipes, the circumferential turning can be avoided to some extent by mounting the scanner so that its center of gravity is below the bottom side of the pipe. However, any devices adapted for preventing the circumferential turning of a scanner employed for a piping laid three dimensionally in the three-dimensional space have not yet been developed.